In the formation of integrated circuits (ICs) it is often necessary to deposit thin material layers or films, such as electrically conductive films containing metal and metalloid elements, upon the surface of a substrate, such as a semiconductor wafer. One purpose of such thin films is to provide conductive and ohmic contacts for the ICs and to yield conductive or barrier layers between the various devices of an IC. For example, a desired film might be applied to the exposed surface of a contact or via hole on an insulating layer of a substrate, with the film passing through the insulating layer to provide plugs of conductive material for the purpose of making electrical connections across the insulating layer and into the underlying layer.
One well known process for depositing such films is chemical vapor deposition (CVD), in which a film is deposited on a substrate using chemical reactions between various constituent or reactant gases, referred to generally as process gases. In CVD, process gases are pumped into the process space of a reaction chamber containing a substrate. The gases react in the process space proximate a surface of the substrate. The reaction results in the deposition of a film of one or more reaction by-products on the surface of the substrate. Other reaction by-products that do not contribute to the desired film on the exposed substrate surfaces are then pumped away or purged by a vacuum system coupled to the reaction chamber.
One variation of the CVD process, which is also widely utilized in IC fabrication, is a plasma-enhanced CVD process or PECVD process, in which one or more of the process gases is ionized with electrical energy into a gas plasma to provide energy to the reaction process. PECVD is a desirable process for lowering the processing temperatures and the amount of thermal energy that are usually necessary for a proper reaction with standard CVD. In PECVD, electrical energy is delivered to the process gas or gases to form and sustain the plasma, and therefore, less thermal energy is needed for the reaction.
Plasma-enhanced CVD uses conductive electrodes within the processing chamber which are biased with radio frequency (RF) energy sources. The electrical fields generated by the RF electrodes activate and sustain the gas plasma. In one configuration, one of the biased electrodes may be the planar susceptor or other structure supporting the substrate to be processed, while the other electrode is a planar showerhead positioned above the susceptor and substrate to introduce the process gas into the processing chamber and simultaneously excite one or more of the process gases into a plasma. Such a configuration is referred to as a parallel plate configuration because of the substantially parallel orientation of the planar susceptor and planar showerhead with the substrate positioned therebetween.
PECVD process chambers using biased showerheads generally support the showerhead above the substrate. An electrically insulative element or insulator is positioned between the biased showerhead and other metal elements in the process chamber. The insulative element is formed of an electrically insulative material, such as quartz. One end of the support is coupled to the metal lid of the chamber, and the other end is coupled to the biased showerhead. In that way, the showerhead is spaced and electrically insulated from the chamber lid. Since the metal lid of the chamber, as well as the metal chamber itself, is electrically grounded, the insulative element prevents electrical contact between the lid and showerhead and this prevents the showerhead from shorting or arcing to ground potential. In one embodiment, the support has a hollow, cylindrical shape and is used to direct the process gas to the showerhead to be dispersed during the PECVD reaction.
Due to the non-selective deposition nature of the PECVD process and the absence of directionality of the actual deposition, films will be deposited elsewhere inside the process chamber other than just on the substrate, as is desired. Specifically, parts inside the process chamber that are in proximity to the deposition area and substrate, i.e., in the vicinity of the electrodes (biased showerhead and substrate support), will be coated. When insulative parts, such as the insulative element, are coated with an electrically conducting film, the plasma and the process stability are detrimentally affected. By such coatings the effective surface areas of the electrodes are extended, and may possibly be extended to create a short to ground of one of the RF electrodes. For example, if the insulative element extending between the grounded chamber lid and the showerhead is coated, the biased showerhead may be shorted to the grounded chamber lid.
Traditionally, there have been several ways to prevent such a problem. However, the conventional solutions have other drawbacks. For example, a ground shield might be used in areas around an RF-biased element to prevent extension of the plasma into those areas and thus to prevent deposition in those areas. Ground shields rely on the Debeye sheath, or dark space sheath, which prevents a plasma from being ignited in certain areas. However, in PECVD processes, the high process pressure of 1-10 Torr requires impractically small shield-to-insulator distances (e.g., 0.1-1 mm). Another method involves frequent cleaning of the process chamber to prevent a build up of a conducting film on the insulative surfaces. However, frequent cleaning has a negative impact on the throughput or productivity of the process. Furthermore, cleaning makes it difficult to then again recover the actual PECVD process, because the PECVD process parameters must again be established anew for the post-cleaning deposition.
Accordingly, it is an objective of the present invention to have a stable plasma in a PECVD process and to prevent shorting of the RF electrodes to ground as a result of non-directional deposition.
It is another objective of the invention to provide such plasma stability while maintaining an efficient and productive PECVD process.
It is still another objective of the invention to achieve the above objectives while eliminating drawbacks associated with traditional PECVD processes.